A MEMS (Micro-Electro-Mechanical-System) device is a micro-sized mechanical structure having electrical circuitry fabricated together with the device by various microfabrication processes mostly derived from integrated circuit fabrication methods. The developments in the field of microelectromechanical systems (MEMS) allow for the bulk production of microelectromechanical mirrors and mirror arrays that can be used in all-optical cross connect switches, 1×N, N×N optical switches, attenuators etc. A number of microelectromechanical mirror arrays have already been built using MEMS production processes and techniques. These arrays have designs that fall into approximately three design categories.
A first category consists of conventional 2D gimbal mirrors with each mirror surrounded by a frame. The conventional 2D gimbal mirror is one of the most common types of MEMS 2D micromirrors. An example is shown in FIG. 6. It consists of a central mirror 10 that is connected to an outer frame 12 with torsional hinges 14. The outer frame 12 is in turn connected to the support structure 16 with another set of torsional hinges 18. There are four electrodes under the central mirror 10 that can be actuated resulting in a 2D tilt of the mirror-frame assembly. One such device is disclosed under U.S. Patent Application Publication No.: US2002/0071169 A1, publication date Jun. 13, 2002. One of the shortcomings of this design is the inability to achieve high fill factors (that is the spacing between two consecutive mirrors or the ratio of the active area to the total area in an array) in a mirror array. An example of a high fill factor would be >90% active mirror portion along one dimension.
A second category consists of 2D/3D mirrors with hidden hinge structures. With significant advances made in Spatial Light Modulators, a number of 2D micromirror devices have been designed with various types of hidden hinge structure. Examples of these are disclosed in U.S. Pat. No. 5,535,047, U.S. Pat. No. 5,661,591, U.S. Pat. No. 6,480,320 B2.
A schematic of an example of such a device is shown in FIG. 7. Although this device structure can yield high fill factor arrays, the fabrication processes are very complex. For more discussion on the Spatial Light Modulators and Digital Mirror devices with hidden hinge structure, references are made to U.S. Pat. No. 5,061,049, U.S. Pat. No. 5,079,545, U.S. Pat. No. 5,105,369, U.S. Pat. No. 5,278,652, U.S. Pat. No. 4,662,746, U.S. Pat. No. 4,710,732, U.S. Pat. No. 4,956,619, U.S. Pat. No. 5,172,262, and U.S. Pat. No. 5,083,857.
A third category consists of 2D mirrors each mounted on a single moving flexible post. An example of a MEMS tilt platform supported by a flexible post 30 as shown in FIG. 8. The post 30 extends within a moat 32 or trench formed in the substrate or supporting material 34. The post 30 can be made sufficiently long and flexible to act as an omnidirectional hinge, bending to allow the mirror 36 to be positioned with two degrees of freedom.
Some of the shortcomings of this design are process complexity, post flexibility, wiring, and tilt eccentricity. A few of such devices have been disclosed in U.S. Pat. No. 5,469,302, US Patent Application Publication No. US 2002/0075554 A1. Furthermore, the control for these devices becomes complex and is a substantial part of the device cost.